It is known that a material densification can occur in the case of amorphous materials as a result of irradiation with electrons. In this regard, a method exists in which electron bonds in the vicinity of the surface are redistributed as a result of the energy input of the electron beam, as a result of which a compaction of the material occurs. This effect can be used for processing optical elements.
By way of example, WO 2011/020 655 A1 describes a homogeneous compaction of the entire surface of a substrate for a mirror or of a substrate already provided with a reflective coating, by irradiation with electrons. The homogeneous densification in the vicinity of the surface brings about a uniform recess of the surface without a significant change of the optical surface shape. This measure makes it possible to prevent a further compaction in partial regions of the mirror by high-energy radiation during a use, for example in a projection lens for microlithography with EUV radiation (radiation in the extreme ultraviolet wavelength range).
DE 10 2012 212 199 A1 furthermore discloses a surface structuring of micro- or nanostructured components composed of glass or ceramic via electron irradiation. To that end, an electron beam having a diameter in the region of the smallest structures to be produced can be directed onto selected partial regions of the surface in order to achieve a local densification and thus a local depression of the surface corresponding to the desired surface structuring. Furthermore, a description is given of processing of an optical element of a projection exposure apparatus for microlithography with an electron beam. Imaging aberrations of the projection exposure apparatus that are caused by aging effects can be compensated for by a suitably implemented densification and an attendant change of the shape of the optical surface of the optical element.
In order to control the electron irradiation, it is conventional practice firstly to determine an energy dose distribution which is to be introduced into the optical element by the irradiation and which is suitable to bring about a desired correction of the surface shape of the optical element on account of material densifications brought about thereby. In accordance with the known information, the energy dose distribution is determined by a simulation of the effect of the incident energy dose distribution on a change of the surface shape. The simulation is based on an assumed linear relationship between a local compaction brought about by the electron irradiation and a surface recess of the optical element that is brought about thereby.
One issue during the processing of surfaces of optical elements by electron irradiation is that the actually achieved shape correction of the surface deviates from the desired shape correction—brought about for example by deformation of the optical element on account of material stresses introduced. This deviation poses problems in particular on account of narrow tolerance predefinitions during the correction of mirror elements of projection exposure apparatuses for EUV lithography.